1. Field of the Invention
The present invention relates to a hydraulic actuating mechanism and, more particularly, to a hydraulic actuating mechanism aiming at energy saving in each hydraulic actuator and a control method therefor.
2. Description of Related Art
A hydraulic die clamping mechanism, which is one of hydraulic actuating mechanisms having been used for a conventional injection molding machine, requires a strong force for die clamping, so that the hydraulic cylinder must have a large diameter, which requires a large amount of hydraulic oil. A hydraulic power source consisting of a hydraulic pump of a constant speed (1200 to 1800 rpm) used for an ordinary hydraulic system is configured so as to deliver the required highest oil pressure and flow rate, and surplus hydraulic oil is returned to an oil tank through a relief valve. Therefore, a great energy loss is yielded.
In order to reduce such an energy loss, some hydraulic systems use a variable flow rate pump. However, the pump of this type produces high noise at the time of constant-speed operation (1200 to 1800 rpm). To solve the above problems, U.S. Pat. No. 3,038,122 has proposed a hydraulic actuating mechanism having a fixed delivery hydraulic pump driven by a variable speed electric motor and a plurality of hydraulic driving mechanisms, wherein the fixed delivery hydraulic pump is a steady feed pump capable of delivering a flow rate and a hydraulic pressure corresponding to the maximum consumption of at least all of the hydraulic driving mechanisms, the rotational speed of the electric motor can be controlled in proportion to the consumption and/or consumed pressure of hydraulic driving mechanisms in operation, and the ramp gradient of change in rotational speed can be controlled by a program incorporated in a controller section.
For an AC inverter motor used for the hydraulic actuating mechanism described in U.S. Pat. No. 3,038,122, acceleration and deceleration of pump are slow due to the inertia of rotor of the motor, so that control such as to increase the speed of an actuator for starting and stopping is difficult to carry out (No matter how fast a change in frequency of inverter may be, it is difficult to follow up the acceleration of motor). Therefore, the position control of a functional part moved by a hydraulic actuator is liable to delay, which presents a problem of overrun. Also, in order to carry out pressure control at the rotational speed of pump, each of the hydraulic driving mechanisms requires a detector for pressure, which makes the hydraulic actuating mechanism expensive.
Also, a plurality of actuators are sometimes operated at the same time (the flow rate and pressure of the main actuator are determined, and for the other actuators, part of hydraulic fluid is branched and circulated, and the pressure is controlled by a pressure reducing valve and the flow rate is throttled by a flow controller). At this time, the discharge pressure of pump must be constant, so that it is necessary to provide a pressure detector at the discharge opening of pump.